Gas hydrates, or simply hydrates, are clathrate compounds, i.e. inclusion complexes, formed by the reaction of molecules of water and another substance such as small hydrocarbon molecules, nitrogen, carbon dioxide and hydrogen sulphide. These molecules are called guest molecules. These stabilize the hydrate structure giving much higher melting temperatures than for ice at elevated pressures.
The melting temperature of hydrates increases relatively fast with pressure. Thus, by way of indication, the melting temperature is typically about 10° C. at a pressure of 20 bars, 15° C. at a pressure of 50 bars, 20° C. at a pressure of 100 bars, and 22° C. at a pressure of 200 bars. Large variations in these values may however exist as a result of variations in fluid composition.
As sea temperatures in places like the North sea, Gulf of Mexico, West Africa and Asia are typically around 4° C. and the pressure that exists in the wells or pipes often lies in the range 50 bars to 300 bars, hydrate formation is a major issue during oil and gas production around the globe as the initially warm, water saturated, fluid from the oil well is cooled in the production pipelines by its surroundings causing condensation which may cause hydrate formation. Production in arctic regions or northern deep water areas increases further the possibilities for hydrate formation due to the even lower ambient temperature. The ambient temperature can in these cases be into the subzero region.
Hydrates can form in all kinds of different pipelines used in oil and gas production and to prevent their formation, it is general practice to insert antifreeze liquids, in particular methanol and MEG (monoethylene glycol), into wells or pipes for the purpose of lowering the melting point of the mixture very significantly.
The antifreeze compounds need to be injected into the main pipeline through chemical injection lines. For operational reasons these lines are equipped with a valve which will be closed when no injection is taking place. Thus the pipe connecting the valve with the main pipeline is forming a section with no flow, i.e. a deadleg. If these deadlegs themselves become blocked by hydrate formation then antifreeze material cannot be injected into the main pipeline and the problems of hydrate formation in the main pipe can lead to shutdown and perhaps into the use of a thruster pig to unblock the pipeline (e.g. see US 2005/0284504). The problem of hydrate formation in deadlegs is also acute as, unlike a main pipeline, there is no constant flow in a deadleg. The constant flow of material through a main pipeline serves, at least in part, to prevent the formation of hydrates as these can be urged through the pipe by the warm flowing oil and gases. In a deadleg where there is no flow, hydrates can therefore form more readily as the conditions therein are more likely to be within the thermodynamic hydrate formation region.
The present invention concerns the prevention of plugging in deadlegs. A deadleg is a closed pipe with no flow which is connected to a main pipe which contains a flowing fluid.
It is critical therefore that the deadleg itself remains blockage free. The problem with deadlegs however, is that they are used only intermittently and are therefore particularly prone to plugging.
Despite the problems of hydrate formation being well documented in main pipelines, the problem of hydrate formation in deadlegs does not appear to be documented in the prior art and no-one appears to have addressed the issue of hydrates blocking deadlegs before, at least in the framework of this present invention. As noted above, the issue is connected to hydrate growth on the deadleg wall due to water condensation on the cold wall of the deadleg as warm gas saturated with water enters the deadleg from the main pipe. Such warm gases can easily be transported over distances of several pipe diameters as a result of mixing cells within the main pipe. These mixing cells are generated by the bypassing flowing fluid in the main pipe. The size and number of these mixing cells depends on the geometric design of the region around the deadleg, the fluid velocities and the gas and liquid content. Humid gas can, however, be transported several pipe diameters into a deadleg. Given sufficient time, the growing hydrate layer may gradually block a cross-section of the whole deadleg or at least create large deposits within the deadleg. This can have dramatic effects on oil production both from a regularity and safety point of view.
Hydrate plugs in a deadleg can represent a high operational safety risk. For example, when a valve in a deadleg is opened, there may be a pressure gradient across the valve or a large pressure gradient can be formed over a possible plug as a result of the subsequent operations. The pressure gradient can loosen the hydrate plug creating a high speed projectile. The consequences of a large lump of ice like material moving through a pipe are potentially enormous with the risk of operator fatalities or at least large material damage. Even the presence of large deposits on the deadleg wall can cause a serious safety hazard. For example, in the case of the deadleg being a part of depressurization system, hydrate deposits may be loosened during the depressurization operation and can plug a valve downstream of the deposit location. As a result of this a high pressure difference is caused which may create a high speed projectile with very serious consequences.
Several solutions exist to avoid hydrate formation in pipes in general. For pipes in onshore or topside installations, heat tracing is often used. Thus, heat can be applied to the pipe by placing a steam line or electric heating element adjacent to the line in the area of plug formation. In practice however, this technique can fail due to malfunctioning of the heating system or due to human error.
A further option for preventing hydrate formation is the use of insulation. Thermal insulation alone may provide sufficient protection against hydrate formation if the length of the deadleg is sufficiently short. No general methodology, however, exists to determine the acceptable length of a deadleg. Design is often done without being aware of the possibility of hydrate plugging. Incidents are known where such hydrate plugs have occurred even in insulated deadleg sections. Other solutions include the application of chemicals. This requires chemical injection points and adds to the cost capital and operational expenditure. In addition due to environmental concerns and product value considerations there is currently a large incentive not to use chemicals.
In the case of deadlegs, the use of chemical injection would also require regular operator action. There always exists a risk that such infrequent operations may be forgotten thus increasing the risk for creating a hydrate problem.
The challenges of hydrate plug removal are exacerbated in subsea systems as lower temperatures generally encourage hydrate formation. The present inventors have found that in some cases plugged deadlegs can be avoided by shortening the deadleg length by installing a valve closer to the warm main pipe. The acceptable distance is determined by complex calculations and can vary from case to case but this technique is limited as it will not always be possible to install a valve close enough to the main pipe to reduce the deadleg length.
Use of insulation can considerably increase the serviceable length of a deadleg but will not eliminate the problem of hydrate plugging for long deadlegs.
More complex techniques for preventing hydrate formation include devices for increasing the heat transfer characteristics of the pipe. These include use of a container built around the deadleg which contains water. Water is warmed up by the warm main pipe, and temperature differences generate convective flow within the container thus warming the deadleg to a larger distance from the main pipe than without such container.
Another solution is the use of a large mass of material with good heat conduction on the outside of the pipe to increase the heat conduction and thus increase the temperature within the pipe.
In principal, these solutions may also be used in surface applications although the cost of their use may be unnecessary in the more accessible surface pipes.
There remains, however, the need to devise further solutions to the formation of hydrate plugs in deadlegs, in particular solutions which are cheap and easily applied to any deadleg whether underwater or on the surface.